Chemical indicator



Reissued Oct. 3, 1950 CHEMICAL INDICATOR Harry Fleisher, Greenbelt, Md.

No Drawing. Original No; 2,416,619, dated February 25,119.47, Serial'No. 549,251, August 12', 1944.; Applicationv for reissue April 11, 1949;

Serial No. 36,854

mom-isle; (01-252408).

(Granted junder the act of March 3; 1883', as. amend'dfApril 30, 1928; 370 O- GQ757) Matter enclosed in heavybrackets;[.lanpears in.the original 'patentbut forms no parrof-thi's reissue specification; matter-printed in .italicsindicatesthe additions made by reissue tween a pH of about 4.6 and 4.9, depending on' solution concentration and temperature.

H'eretofore,. practically all volumetric tests for determining the alkalinity of solutions; and particularly of waters, were conducted-with the aid of methyl orange as the indicator. Howeverythew maximum color transition of methyl "orange occurs at pH values rangingfrom about 3.8-tn 4.3, which makes the indicator difficultto'use in'detecting an end-point lying in the pH' range 4.6

to 4.9.even in the hands of. an experienced 'operator. For this reason, numerous attempts have been made by various investigators to develop indicators which would be suitable'for use in place of methyl orange. These attempts have thus-far failed because the indicators developed therefrom were subject to one or more handicaps orliinit'ations, including little or no improvement" over the methyl orange color change; failure to accurately identif the true equivalence point of the neutralization reaction; lack 'ofsolution stability,

often due to the alcoholic vehicle used; complexity of composition resulting. in increasedcost; and limited applicabilit because of'incompatibility with contaminating elements;

It is accordingly an object of the-present in vention to provide a stable and economical indicator capable of wide application by virtue'of its resistance to interference from contaminating elements. Another object is to provide an "indicator which possesses a colortransition ofsuch' distinct contrast as to make the. end-point-unmistakably distinguishable. A furtherobje'ct'is to provide an indicator which possesses a color transition end-point practically"coinci'dentfwith the equivalence point 'encountered 'in water-ab kalinity analysis;

The above objectives are accomplishedaccording to the present invention by the *use-of solutions of the sodium salt of dimethylaminoazohenzene-o-carboxylic acidtowhichare-adde'd various I 2 coloring agents in proper ratio to insure maximumiaccuracyv'and sensitivity. Thez-sodiumsalt of dimethylaminoazobenzene-o-carboxylicncid is in itself a water soluble acid-base indicatonsshowing color transitions "of yellow to .red" in passing from the alkaline to the acid state. However, this change israther-difiicult .to detect and .furthermore occurs at a pH value not: within/the limits previously established forvthe neutralization reaction, i. e;, 4.6 to 4.9. I have-discovered that achromatic effects induced by thescombination of dissimilar;.colorsimake the indica-torcolor transition more readily detectable, and further, cause the said transition'to OCCl11.WithiIITthEDH range 4.6 to 4.9. For example, the color-produced by a-blue dye supplements the-'colorsproduced by the indicator during its transition: to

give complementary shades of green to:viol.e-t,

which display a distinct: contrast. at '"theven'dpoint of thereaction; In this .case'the violet color appears abruptly with the reaching of the equivalence point involved in the neutralization reaction employed in' water alkalinity analysis. This is usuallya pH of about4.89 forrboilenfeed waters, though, as stated above; the-rpHI atthe equivalence point may range from about 4-5" to 4.9 for most water alkalinity analyses- The following specific examples will further illustrate the invention. It isunderstoodthat: the invention is'not limited thereto sincevariatio'ns and substitutions of dyestuffs or'coloring' agents, and of-quantities of indicator and dyestuffjii are possible within the spirit of the invention. The various colourindex numbers given belowanrltin the'appended claims refer to coloncompounds hearing such numbers as described in"ColourIndex, (1924); Society of Dyers and Golourists,

Yorkshire, England."

Example I.

0.45 gram of sodium salt of dimethylaminoazobenzene-o-carboxylic acid is dissolved in' 'about 300 cc. of distilled'water. To this solution is added 0.55 gram of a blue dyestu-fi" (Colour 'Index No. 714), and the mixture stirred to effect solution of the dye. Sufficient distilled.water"is then added toxthesolution' to makeatotalwolume of 1000 cc;

Example II...

0.55 gram of sodium salt of dim'ethylaminoazo benzene-o-carboxylic acid is dissolved inqabout 300 cc. ofdi'stilled. water." To *this'soluti'on is added 0.50 gram of a bluerdyestuff (Colour Index No. 707), and-the mixture stirred-:toefiect solution of the dye. Sufficient distilled water is then added to the solution to make a total volume of 1000 Example III 0.50 gram of sodium salt of di methylaminoazo benzene-o-carboxylic acid is dissolved in about 300 cc. of distilled water. To this solution is added 0.40 gram of a blue dystuff (Colour Index No. 833), and the mixture stirred to effect solution of the dye. Sufficient distilled water is then added to the solution to make a total volume of 1000 cc.

Example IV 0.45 gram of sodium salt of dimethylaminoazobenzene-o-carboxylic acid-is dissolved in about 300 ccqof distilled water. To this solution is add? ed 0.65 gram of a blue dyestufi (Colourlndex No. 518), and themixture stirred to effect solution of the dye. Sufiicient distilled Water is then added to the solution to make a total volume of 1000 In applying the indicator solutions to the determination of water alkalinity, the usual practice is to add three drops of the indicator to each 100 cc. of water being tested. Titrations are usually conducted atroom temperatures 70-80 F.), although accuracy of the titration is not materially afiected by the usual variations from room temperature. In alkaline solutions, the color produced b the indicator solutions is green. As the titration progresses the green color disappears and the solution becomes either light-gray or colorless. This change provides the operator with adequate warning of the approaching endpoint. Upon'the completion of the neutralization, the indicator imparts a violet hue to the liquid, whichcolor is takenas the end-point for the titration;

" Laboratory check tests have ascertained that this end-point distinctly identifies the equiva-.

'lence point for the neutralization reaction involved in water alkalinity analysis with a 'high degree of accuracy. The efiect of interfering elements such as phosphates, neutral salts and organic matter on indicator accuracy has also been checked and found to be negligible. Service tests have proven the solutions to be stable for periods "greater than one year. However, in the light of this information, it is not unreasonable to believe that the solutions possess indefinite stability under normal conditions.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

Within the scope of the appended claims, various modifications and changes in the nature and proportions of' the ingredients may be made by those skilled in the art Without departing from the nature of the invention.

[lpAn indicator, characterized by the ability to indicate the end point of a neutralizing reaction in thedetermination of the alkalinity in water,

' consisting of a mixture of the sodium salt of dimethylaminoazcbenzene-o-carboxylic acid and a water soluble blue dyestufi, the ratio of the dyestufi tothe sodium salt i said mixture being such as to indicate the said end point at a pH range of 4.6to 4.9.] 1

[2. An indicator, characterized by the ability to indicate the end point'of-a neutralizing reaction in the determination of the alkalinity in .zwjater consisting of a mixture of the sodium salt consisting of a mixture of the sodium salt of dimethylarninoazobenzene-o-carboxylic acid and a water soluble dyestufi" bearing Color Index Number 707, the ratio of the'dyestuff to the sodium salt in said mixture being such as to indicate the said end point at a pH range of 4.6 to 4.9.]

[4. An indicator, characterized by the ability to indicate the end point of a neutralizing reaction in the determination of the alkalinit in water consisting of amixture of the sodium salt of dimethylaminoazobenzene-o-carboxylic acid and a Water soluble dyestuff bearing Color Index Number 714, the ratio of the dyestufi to the sodium salt in said mixture being such as to indicate the said end point at a. pH range of 4.6 to 4.9.]

5. An indicator, characterized by the ability'to indicate the equivalence point of a neutralizing reaction in the determination of the alkalinity in water, consisting of a mixture of the sodium salt of dimethylamiuoaeobeueeue-o-carboarylic acid and a water soluble blue dycstufi, the ratio of the dyestufi to the sodium salt in said mixture being such-as to indicate substantially the equivalence point by the passage of the color of the mixture' through a predeierminedly-arrauged sequence of at least three distinct and contrasting color phases, one of which has a colorless or grayish hue and occurs immediately before the minedly-ar'rangcd sequence. of at least three dis tinct and contrasting color phases of green, light gray or colorless, and violet, two of which begin before the equivalence point and one of ea gins substantially at the equivalence point.

7. An indicator, characterized by the ability to indicate the equivalence poiut of a neutralizing reaction in the determination of thealhaliuity in water, cousistiugof a mixture of the sodium salt of dimethylamiuoaaobeneene-o-carborylic acid and a water soluble dyestufi bearing Colour Index Number 714, the ratio of the dyestufi to the sodium salt in said mixture being such as to indicate substantially the equivalence point by the passage of the color of the'miaturc through a predcten miuedly-arranged sequence of at least three distinct and contrasting color phases of green, light gray or colorless, and violet, two of which begin bcfore the equivalence point and one of which and a water soluble dyestufi bearing Colour Index Number 707, the ratio of the dyestufi to the sodium salt in said mircture bein such as to indicate substantially the equivalence point by the passage of the color of the mixture through a predeterminedly-arranged sequence of at least three distinct and contrasting color phases of green, light gray or colorless, and violet, two of which begin before the equivalence point and one of which begins substantially at the equivalence point.

9. An indicator, characterized by the ability to indicate the equivalence point of a neutralizing reaction in the determination of the alkalinity in water, consisting of a mixture of the sodium salt of dimethylaminoazobenzene-o-carbo:cylic acid and a water soluble blue dyestufi, the ratio of the dyestufi to the sodium salt in said mixture being such whereby the color of the mixture during the neutralizing reaction passes sequentially through a greenish hue, a grayish hue or colorless, to a violet hue, the final color beginning substantially with the equivalence point.

10. An indicator, characterized by the ability to indicate the equivalence point of a neutralizing reaction in boiler water or boiler feed water, o0nsisting of a mixture of the sodium salt of dimeth- 6 ylaminoazobenzene-o-carboaylic acid, a water soluble blue dyestufi and distilled water in substantially the relative proportions of 0.45 to 0.55 gram of sodium salt, 0.40 to 0.65 gram of blue dyestufi and the remainder of distilled water to complete 1000 c. c; of the mixture whereby the same during the neutralizing reaction in the boiler water or boiler feed water passes through a predeterminedly-arranged sequence of at least three distinct, readily recognizable, and contrasting color phases, at least two of which begin before the equivalence point and one of which begins substantially coincidental with the equivalence point.

HARRY FLEISI-IER.

REFERENCES CITED The following references are of record in the file of this patent or the original :patent: 

